drm/radeon: add WAIT_UNTIL to evergreen VM safe reg list

It's used in a recent mesa commit:
http://cgit.freedesktop.org/mesa/mesa/commit/?id=24b1206ab2dcd506aaac3ef656aebc8bc20cd27a


and there may be some other cases in the future where it's required.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Cc: stable@vger.kernel.org